1. Field of the Invention
The present invention relates to a pneumatic tire which can discharge a static electricity generated in a vehicle body or a tire to a road surface.
2. Description of the Related Art
Recently, there have been proposed a pneumatic tire including a tread rubber blended with silica at a high ratio in order to reduce the rolling resistance that largely affects the fuel consumption of a vehicle and/or to increase braking performance (WET braking performance) on a wet road surface. However, compared to a tread rubber blended with carbon black at a high ratio, the electric resistance of such tread rubber is high, and accordingly, static charge generated on a vehicle body or the tire is prevented from being released to the road surface. As a result, problems like radio noises tend to occur.
There has been developed a pneumatic tire provided with a solution of the electric resistance problem in such a manner that a conductive portion blended with carbon black or the like is formed within a non-conductive tread rubber blended with silica or the like. For example, in pneumatic tires disclosed in Japanese Unexamined Patent Publication No. 10-81110, a non-conductive tread rubber is formed with a conductive portion extending in a diameter direction of the tire to release the static charge to the road surface through the conductive portion. The conductive portion extends from a ground surface to a bottom surface through the tread rubber and constitutes a conductive path for releasing the static charge.
In a pneumatic tire described in the following Japanese Unexamined Patent Publication No. 2009-126291 and WO 2009/066605, a non-conductive tread rubber is provided with a conductive portion extending in a tire width direction, for the purpose of preventing a uniformity from being deteriorated. The conductive portion extends in the tire width direction between a cap portion and a base portion from a side wall rubber or a topping rubber of a carcass which electrically conducts with a rim, and is inclined in the vicinity of a tire equator so as to be exposed to a ground surface. However, in this structure, it has been known that an ununiformity of rigidity is caused in the tread rubber, and an irregular wear tends to be generated.
In other words, in the tread rubber mentioned above, it has been known that a peripheral position of the tire equator having the conductive portions being comparatively closely formed and coming close to the ground surface wears preferentially, and a so-called center wear tends to be generated. Further, in the pneumatic tire described in Japanese Unexamined Patent Publication No. 2009-161070 filed by the present applicant, there is employed such a structure that the conductive portion is branched, in order to securely achieve a conductive performance from an early stage of the wear, and in the structure mentioned above, there is a risk that the irregular wear is remarkably generated since a degree at which the conductive portions are closely formed is increased.